Few wish to examine the possibility that the killer may simply have taken to a violent extreme ideas that, while hateful, are current in mainstream political debate

The language of lunacy is political. When I first heard that Islamist terrorists had slaughtered up to 80 innocent people in Norway, I was at a gathering of left-wing activists – the sort of people who are ordinarily supposed to question knee-jerk assumptions about political butchery and the Islamic faith. Very few of us did. "It's because they reprinted that Danish cartoon," someone said, reading from the newswires. We nodded solemnly. We did not think to ask whether the ethnicity and ideology of the killers had yet been confirmed. And nobody used the word "madman".

The next day, even as The Sun's headline screeched that "Al-Qa'ida" had launched "Norway's 7/7", it emerged that everyone's first guess had been wildly wrong. Not only is the man responsible for the monstrous attacks in Utøya and Oslo a white, Christian European, he is also a fanatical anti-Islamist, claiming common ground and possible direct links with far-right groups across Europe, including Britain. Immediately, there was an embarrassed scramble to change the rhetoric. No longer was the killer a criminal mastermind, part of a sinister "terror" network – he was simply a "madman", a "psycho", a "lone nutter". Where few had paused to question the pernicious, organised frenzy supposedly driving any Islamist assailant to acts of slaughter, these crimes were clearly the product of a deranged mind, acting alone, and by that logic had absolutely nothing at all to do with the rise of far-right extremism in Europe and of popular Islamophobia across the West.

Calling someone mad has always been a way of "othering" them, of separating them from the rest of society. From sexually promiscuous women in the late 19th century to homosexuals in the 1950s, social deviance has been medicalised in order to punish behaviour deemed loathsome or intolerable, without placing any duty upon society to respond to it as part of its own culture. So tenacious is the taboo of "madness" that people who really do have mental health difficulties still suffer discrimination in their communities and workplaces.

Up to a quarter of the adult population, according to WTO figures, has some experience of mental health difficulty, and most of them do not make a habit of mass murder. Some other explanation is required. It is to their credit the dignified men and women of the Norwegian Intelligence Service continue to resist the easy dismissal of the "lunatic" charge, insisting that their home-grown killer was "evil, not mad".

I am not especially interested in the intimate psychology of Anders Breivik. It goes without saying that there is something horrifyingly wrong with anyone, whatever their ethnic and ideological background, who thinks that butchering teenagers is an appropriate response to political expediency. Breivik's precise mental robustness is of importance to nobody but his defence lawyers. What matters more is the speed with which so many have rushed to claim, and loudly, that he must be mad, as if madness automatically placed one outside the realm of political and personal responsibility.

Significant parts of the political establishment seem frantic to believe, as Sam Leith wrote in the Evening Standard, that "Norway was about madness, not politics"; that the events in Oslo and Utøya were "so exceptional as to be of interest to criminology and brain science, but not to politics", in the words of Simon Jenkins in The Guardian. Few have been prepared to examine the far more frightening possibility that the killer may simply have taken to a violent, bloody extreme ideas that, while hateful, are entirely current in mainstream political debate.

A glance through Breivik's exhaustive 1,500- page manifesto, in which he lays out his plan to free Europe from the scourge of "Islamification" and "multiculturalism", shows that this was not the work of someone in the throes of mental breakdown. The document describes its dialectic of fear with a cold, well-researched reserve that is infinitely more chilling than any foam-flecked fascist rant. Much has been made of the manifesto's absurdist imagining of a new order of "Knights Templar" to purge the continent of Muslims, but the sillier parts of this grimoire of hate are not the most disturbing: the bulk of the text echoes a rhetoric of prejudice and social suspicion that has become so routine a part of public discourse in the West that several prominent, respected British and American commentators are quoted at length.

To claim that this massacre was "nothing to do with politics" is offensive as well as illogical. Not only is there written evidence of an ideology, and a terrifyingly familiar one, behind these attacks, the claim insults the memory of the 68 young activists from the Norwegian Labour Party who lost their lives because of their political beliefs, because a right-wing terrorist had decided that, as the next generation of "Category A traitors" – supporters of "multiculturalism" and "feminism" – they deserved to die.

Writing off Anders Breivik as a "lone nutter" allows us to decry his actions without dismissing his ideas. This is precisely what Stephen Lennon, the leader of the far-right English Defence League, attempted to do in an interview on Newsnight this week, when it was put to him that Breivik listed several hundred EDL members as online contacts and had posted a message of support to the group shortly before disappearing to carry out his attacks. After expressing his sympathy with the people of Norway, Lennon told Jeremy Paxman that he shared Breivik's belief that Islam was "a threat", and that millions of people across Europe felt the same. "You can't just brush off millions of people who have concerns against Islam as lunatics," he said. "You need to listen to us, because god forbid this ever happens on English soil... that's not a threat, it's a wake-up call."

The real lunacy is that there are many in the political mainstream who would agree with him. As governments across Europe have failed to offer any coherent response to the disenfranchisement of the working class, it has become routine to pander instead to the ugliest prejudices of those whose jobs and livelihoods are at risk from the stagnation of wages and the collapse of secure housing and employment. With the far-right gathering in strength and confidence, we can no longer afford to indulge the ideology of hate that led to the Norwegian massacres. That way true madness lies.